An evertable drum centrifuge has a housing extending along an axis and defining a back filtrate compartment and a front solids compartment spaced therefrom and a filter drum rotatable in the housing about the axis. This drum has a generally cylindrical and perforate outer wall centered on the axis, axially fixed in the filtrate compartment, and having an axially forwardly open front end. An end wall of the drum extends transverse to the axis and is displaceable axially between a closed position fitting in and closing the front outer-wall end and an open position spaced axially forward of the outer wall and lying in the solids compartment. This end wall is formed at the axis with a central fill opening. Extension structure fixed on the end wall has an annular rim spaced axially backward from the end wall and an annular liner of a flexible foraminous filter medium has a front edge attached to the outer-wall end and a back edge attached to the extension-structure rim. The end wall and extension can be displaced between the closed and open positions to displace the liner from a normal position inside the drum and extending backward from the front end to the rim to an everted position substantially outside the drum and extending forward in the solids compartment from the front end to the rim. A fill tube extending along the axis and having a back end provided with a fill head engageable in the fill opening can feed a suspension to an interior of the drum.
Thus in the normal position of the drum solids will be trapped as a filter cake on the liner and the liquid of the suspension will pass through the the liner and through perforate drum wall to the filtrate compartment. To clear the filter cake the end wall and structure are moved into the front position so that the liner is everted, that is turned inside out, in the solids compartment. The cake on the liner falls off, if necessary helped by rotation of the drum and liner. Such a system can therefore be used for batch filtering.
In German patent document 3,740,411 of H. Gerteis the fill tube is axially fixed in the housing and the fill hole of the end wall is provided with a liner sleeve supported on a bearing for rotation about the axis and also provided with flexible seals engageable with the head of the fill tube. The fill-tube head in turn is enlarged and has axially oppositely tapering ends. Thus as the end wall is moved to the open position it slips over the head. This system is an improvement on the system of earlier German patent 3,430,507 of H. Gerteis where the fill tube is also stationary but fits with play through the fill hole.
The problem with this system is that there is invariably some leakage between the head and the end wall. The filtrate is splashed back by the structure. Thus some of the filtrate gets into the solids compartment. As a result the stripped-off filter cake is moistened and contaminated. Furthermore in a system where the interior of the drum is pressurized to assist the filtration, leakage at the joint either makes such pressurization very difficult or altogether impossible.
In German patent 3,916,266 of H. Gerteis the fill hole is provided with a fluid-pressurizable gland/seal. Such a system works wholly batch wise, that is the seal is opened and the fill tube is pushed through the fill opening into the drum, a quantity of suspension to be filtered is injected into the drum, and then the fill tube is retracted and the opening closed by the gland so that the drum can be rotated and the suspension filtered.
This system largely eliminates any possibility of leakage, but is slow and cumbersome to use. The retraction and advance of the fill tube is an extra step that unnecessarily slows production, and each batch can only be as much as will fill the drum to slightly less than half full, so a succession of small batches must be processed before the filter cake is turned out.
In PCT publication WO 92/04,982 also of H. Gerteis the fill tube can rotate and passes through a fairly tight pressurizable gland carried on the fill tube into the drum. During filling and operation thereafter the joint between the fill tube and the drum end wall can therefore be sealed very tightly. Such a system is, however, fairly complex with respect to feeding the suspension to the drum and pressurizing the gland. In addition a separate drive must normally be provided for the fill tube as otherwise the gland is subjected to damaging torque.